232 BRIEF OUTLINE OF VEGETABLE PHYSIOLOGY 



soil water. Sulphur, phosphorus, potassium, etc., are obtained in the 

 form of salts from the soil. 



541. As regards the number of elements supplied, the root is 

 therefore the chief organ of absorption ; the leaf absorbs only carbonic 

 acid gas. ^ Absorption at the root may be considered under two 

 heads : absorption of water, and absorption of nutrient salts. 



542. Absorption of water. — The manner in which the root sends 

 out root hairs, wliich become applied to the soil particles for the 

 purpose of absorption, has been described in an earlier chapter. What 

 force acts to draw water into the root hairs is not known with 

 certainty. It is believed by most physiologists to be the osmotic force 

 of the root hair cells (see page 230, footnote). 



543. Aside from the scarcity or abundance of water in the soil, 

 the chief external circumstance affecting the rate of absorption is that 

 of temperature. Warmth increases absorptive activity, while cold 

 decreases, or even prohibits it. Sachs found that at a temperature of 

 from 38° to 41° F. absorption of water ceased, in spite of the fact 

 that the soil was saturated. 



544. Absorption of nutrient salts. — The salts needed for perfect 

 nutrition may be swept into the plant in the absorption current. In 

 case the salts are bound by adhesive force to the soil particles, they 

 must first be loosened by the action of acids excreted by the root 

 hairs. When they exist in free solution in the soil water, or have 

 been brought into this condition by the secretions, they may pass into 

 the root hair quite independently of any current, by the process 

 known as diffusion. The dissolved particles of the salt wander 

 throughout the body of water in which they find themselves, through 

 the root-hair walls, and so on through the tissues of the plant body, 

 unless they meet membranes possessing pores too minute to allow 

 of their entrance. Those salts that are most used by the active cells 

 and are therefore scarcest in the general sa]3 of the plant, diffuse 

 from the soil into the plant more rapidly than those that are little 

 used and that therefore tend to become concentrated in the sap. 

 Each kind of plant, according to its nature, by internally appropri- 

 ating more or less of this or that salt, thus controls the absorp- 

 tion of the different soil salts at the root. Some kinds of plants 

 tend to exhaust one constituent of the soil, some kinds another con- 

 stituent. Plants are therefore said to show selective absorption of 

 nutrient salts. 



545. The transfer of water through the root and stem to the leaf 

 is accomplished by a number of forces. In the case of deciduous trees 



1 Like all other parts of the plant, the leaf absorbs oxygen for respira^ 

 tion. But we are here considering the raw materials from which food is 

 formed. 



